LIFE OF BACTERIA 



257 



quiet air. Why? That they stick to particles of dust can be 

 proved by placing a little dust from the schoolroom on a culture 

 medium. Bacteria are present in great numbers where crowds of 

 people live and move. The air from dusty streets of a populous 

 city contains more bacteria than does the cleaner air of a village 

 street. The air of a city park contains relatively few bacteria 

 when compared with the air of a near-by street; the air of the 

 woods or high mountains, fewer still. Why? Our previous 

 experiments have shov/n that 

 dirt on our hands, the mouth 

 and teeth, decayed meat and 

 vegetables, dirty money, the 

 very hairs of one's head, are 

 all carriers of bacteria. 



Fluids the Favorite Home 

 of Bacteria.^ — Tap water, 

 standing water, milk, vinegar, 

 wine, cider, all can be proved 

 to contain bacteria by experi- 

 ments similar to those already 

 suggested. Spring or artesian 

 well water would have very 

 few, if any, bacteria, while 

 the same quantity of river 

 water, if it held any sewage, 



might contain untold millions of these little organisms. Moisture 

 is absolutely necessary for bacterial growth ; consequently, keeping 

 material dry will prevent the growth of germs. Bacteria grow 

 most freely in fluids. 



Food of Bacteria. — Bacteria are living and contain no chloro- 

 phyll, and we should expect them to need protein food in order 

 to grow. Such is not always the case, for some bacteria seem 

 to be able to build up protein out of simple inorganic nitro- 

 genous substances. If, however, we take several food sub- 

 stances, some containing much protein and others not so much, 

 we find that the bacteria cause decay in the proteins almost at 

 once, while other food substances are not always attacked by 

 them. 



Growth of bacteria in a drop of impure 

 water allowed to run down a sterilized cul- 

 ture in a dish. 



